Semiconductor devices having heterojunctions are of considerable interest today for use in, for example, lasers, light-emitting diodes, bipolar and field effect transistors, and photodetectors as well as other types of devices. A heterojunction is a junction between two different semiconductors which may have either the same or opposite conductivity types. Semiconductor heterojunction devices may be usefully fabricated with more than one heterojunction. For example, a typical semiconductor laser in commercial use at the present time has two heterojunctions. Devices may be fabricated with numerous heterojunctions formed by interleaving layers of two different semiconductors. If the layers are ultra-thin, i.e., quantum size effects are significant, the resulting structure is commonly termed a "superlattice" and may be used in many types of devices. One type of device is of interest because it has interesting transport porperties, such as, for example, negative differential resistance. See, IBM Journal of Research and Development, 14, pp. 61-65, 1970. Alternatively, the superlattice could be produced by varying the conductivity type. Thus, superlattices may be formed without a heterojunction.
Superlattice structures having a spatially varying potential produced by thin, highly doped regions of alternating conductivity types separated by regions of intrinsic conductivity have also been disclosed. Such a structure is commonly termed a "nipi" structure and is described in, for example, Phys. Stat. Sol., 52, pp. 79-92 and pp. 533-544, 1972. Also, see U.S. Pat. No. 3,882,533 issued on May 6, 1975 to G. H. Dohler. Although the optical and electrical properties, including carrier transport properties, are varied by the disclosed structure from the properties of the undoped structure, the described devices use only a single semiconductor material.
Other techniques have been proposed to form a superlattice and modify carrier transport properties. For example, the use of ultrasonic waves to form spatially periodic variations in the carrier conduction properties is disclosed in Soviet Physics Solid State, pp. 1658-1659, August 1962. Use of the structure as, for example, a high frequency oscillator is suggested.
Yet another use of charge sheets, which may also be referred to as planar doped charge sheets, is described in IEEE Electron Device Letters, EDL-3, pp. 407-408, December 1982. A charge sheet is a highly doped region having a thickness in one direction, which may be either parallel or perpendicular to the carrier transport direction, that is small compared to the two other spatial dimensions of the sheet. The structure described is termed a "repeated velocity overshoot structure" and utilizes nonsteady-state electron transport. This type of transport is obtained by using planar doped charge sheets having alternating conductivity types located within a region of intrinsic conductivity which, in turn, is located between n- and p-type regions. The dipole field of the pair of charge sheets shifts the electron energy distribution to higher energies and thereby enables velocity overshoot to occur repeatedly. Also disclosed is the use of a graded bandgap repeated overshoot device.
Use of a single planar doped layer in an intrinsic conductivity region sandwiched between two highly doped, identical conductivity type regions in a device is described in Electronics Letters, 16, pp. 836-838, Oct. 23, 1980. This structure, termed a "planar doped barrier", permits asymmetric current-voltage characteristics to be obtained. Variations in the barrier position and the charge density in the barrier permit different barrier heights and positions to be obtained.
In many heterojunction device applications, the energy band discontinuities, detract from the desired carrier transport properties when charges are trapped at or near the interface because they lack sufficient energy to surmount the energy barrier. Charge trapping is undesirable in, for example, photodetectors because it adversely affects the device response time. For this as well as for other reasons, the concept of graded bandgap structures, i.e., the semiconductor composition is varied in a manner such that the energy bandgap is also varied, has been introduced to desirably modify device properties. Such graded bandgap structures are useful in many applications. See, for example, Applied Physics Letters, 36, pp. 373-376, Mar. 1, 1980, which discusses a rectifying structure. See also, for example, IEEE Transactions on Electron Devices, ED-30, pp. 381-390, April 1983, which discusses avalanche photodetectors. U.S. Pat. No. 4,476,477 issued on Oct. 9, 1984 to F. Capasso, W. T. Tsang, and G. F. Williams, describes the use of charge sheets having alternating conductivity types positioned in a graded bandgap region near a heterojunction having an abrupt stepback to a lower bandgap to increase the carrier ionization rate in an avalanche photodetector. The charge sheets are desirably positioned so that their effect is to increase the carrier energy. While graded bandgaps have many advantageous applications, they may be undesirable in some applications because they do not always preserve the abruptness of the heterojunction. Of course, in other devices such as field effect transistors, the energy band discontinuity is desirable because it provides carrier confinement.
Detailed consideration of the discussed reference indicates that they typically consider the energy band discontinuities, as well as the barrier heights, in heterojunction devices as unalterable characteristics of either the device or the semiconductor heterojunction. There has been some speculation that growth conditions, substrate orientation and even defects near the interface may have an effect on heterojunction properties. See, for example, Surface Science, 132, pp. 456-464 (Zur et al), 469-478 (Margaritondo), 513-518 (Waldrop et al) and 543-576 (Kroemer), 1983. In particular, Zur et al suggest that a dipole formed by defects of opposite charge might be used to vary the heterojunction band offsets. However, detailed knowledge, or even consensus, among those skilled in the art, of the parameters that would enable the heterojunction characteristics to be controllably varied is lacking.